Process for the manufacture of cold-rolled steel strip with superior mechanical workability, especially deep forming properties

ABSTRACT

Process for the manufacture of cold-rolled steel sheets with superior mechanical workability, especially deep-forming qualities, comprising subjecting cold-rolled steel strips to a preliminary annealing step at a temperature below the A1transformation point until the carbon content of said strips is reduced to less than 0.06%, and further subjecting said strips to a second annealing step at a temperature between the A1transformation point and the A3-transformation point.

W II HQ 1131 States Patent Eubotera et a1.

PROCESS FOR THE MANUFACTURE OE COLD-ROLLED STEEL STRIP WITH SUPERIOR MECHANICAL WORKAEILITY, ESPECIALLY DEEP FORMING PROPERTIES Inventors: lllaruo Kubotera; Kazuhide Nakaoka; Kaoru Watanabe; Teruhisa Tanaka, all of Kawasakiski, Japan Assignee: Nippon Kokan Kabushiki Kaisha, Tokyo,

Japan Filed: Nov. 12, 1969 Appl. No.: 871,559

Related U.S. Application Data Continuation of Ser. No. 567,334, July 20, 1966, abandoned.

Foreign Application Priority Data Aug. 10, 1965 Japan ..40/48225 U.S. Cl ..148/2,l48/12.1,148/16 Int. Cl. ..C21d 1/74 Field of Search ..l48/2,3, 12.1, 16,39,134

[15] 3,657,022 [451 Apr, 1%, 1972 Primary Examiner-Charles N. Lovell Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak [5 7] ABSTRACT Process. for the manufacture of cold-rolled steel sheets with superior mechanical workability, especially deep-forming qualities, comprising subjecting cold-rolled steel strips to a preliminary annealing step at a temperature below the A,- transformation point until the carbon content of said strips is reduced to less than 0.06%, and further subjecting said strips to a second annealing step at a temperature between the A,- transformation point and the A -transformation point.

1 Claim, 7 Drawing Figures P'A'TE'NTEI] APR 1 a 1912 SHEET 10F 2 1&1

When an example of steel charge A, briefly to be denoted hereinafter throughout the present specification and having a composition as set forth in the following Table 1(a), was subjected as conventionally, upon solidified, to a coldrolling and a Thi a lication i a ontinuation of a li tion, S N 5 final annealing, the thus obtained sheets provided mechanical 567,334,filed July 20, l966,now abandoned. properties as enlisted in Table l (b), a microscopic photo- The present invention relates to an improved process for the g ap of the Products being Show" in manufacture of cold-rolled steel sheets with superior mechani- In FIG. l-(a), the results were obtained from the sheet upon cal workability, especially deep-pressing qualities. annealed in the normal way at 700 C. FIG. l-(b) and (c) The main object of the present invention is to provide an show the results of the sheet upon subjected to a decarburizing improved process of the above kind, for the manufacture annealing at 700Cand 800 C, respectively. rimmed steel base sheet products, yet having a superior deep- From these results, it can be seen that the mechanical proforrning machinability, in a highly simplified and economical perties of the steel sheets can be appreciably improved by the manner. decarburizing treatment, a considerable amount of finer These and further objects, novel features and advantages grains still remains in spite the higher temperature annealing will become apparent as the description proceeds in reference such as, for instance, 800 C, which means th t a certain to h mp y g ng in which: sure is to be adopted for carrying out for letting these finer FIGS. 1 7 represents reproduced microscopic photograins to growth to a maximum allowable limit, so as to imgraphs, x 100, of several examples of structure of steel sheets prove till f h r th ech i l properties f h m r obtained y the conventional n novel processes- Upon profound experimental and theoretical investigation A conventionally acknowledged measure to improve the i h way f improving th growth f crystal grains i h press-forming workability of rimmed steel sheets is to reduce course f h d b i i annealing f 1d- |l d sheet the carbon in Steel to a reasonable Value- The Weilproducts, it was now found that the mechanism in this sense is acknowledged reason y those Skilled in the art is to reduce decidedly influenced by the existence of minor interlattice the q y of carbide and Solid solute Carbon in Steel, particles having particle dimensions in the order of In. or less because an excessive amount of these substances in steel will and that when the unit number f these minor particles is affect "P the steel ht hardening thereof P solidified reduced intentionally, the desired mechanical properties of that the maehihahihtyr p e y the pressformmg h the sheet products can be substantially improved. As ascerteristic of the steel products, IS considerably affected in the mined f experiments, these minor particles are oxides negative sense. When the carbon content is reduced, the carfide or the like, predominantly however the form There, bide in steel will first be reduced and then the solid solute carfore, it may be well expected from the above observation and hon in F thereby steel becoming horrespqhdihgly consideration that by reducing the oxygen content in molten softer, while at the same time the crystal grains are liable to Steel to an allowable minimum so as not to adversely affect growth. With the growth of the crystal grams, the steel WIII upon the desirous rimming Step, the desired purpose could become still further softer and the crystalline aggregate as a Substantially be fulfilled whole Whl Fh ih its hf ih the heheht of improving the Various different measures for reducing the oxygen content deep'machlhmg workability of the Steel in steel may be conceivable, of which a preferable, economi- As a commqnly employed measurefqr the reduguon of cal, easy and simple one may be that which utilizes the balancboh content m Steel the dechrbunzmg ahneahng prohess ing condition of oxygen with carbon, manganese or the both. generally known as the open coll processigenerhny h 40 A measure for this purpose is shown in Table 2(a) and 2(b), the steel sheets prepared and conditioned m this way wherein the carbon content was controlled so as to reduce the .represhms carbon content of 0.003% or iheheby h oxygen content in steel. The former table specifies an example g a highly i workability m companshn of steel charge composition, while the latter table enlists the 3:2 ggi ig g sgg s fig rhs gggzig 2 22:213 32 22: 3: mechanical properties, by way of example, of the steel sheet highly difficult, as ascertained from our profound experimen- B2 .Fh lhe he!P.th ekf h 'fhih phocess tal knowledge and experience, to provide steel sheets, capable of representation substantially none of surface defects upon TABLE 2(a)' CHEMICAL COMPOSITION subjected to a deep-pressing deformation, and having rather Charge 0 Mn P s 02 N2 coarse, yet substantially in an ordered grain size such as, for 12 M0 M15 M21 M29 M012 instance, A.S.T.M.No. 6, allowable within the limits for the g i g gifghihahoh Said a-hoyhr e A accompanied by a decarburizing annealing at 750" C. A corresponding microscopic photograph is shown in FIG. 2, as an TABLE 1(8) 5 5 example. In comparison of these results with those give in the he Composition of Steel Charge foregoing Table 1(a), it will clearly be acknowledged that in Charge 0 Mn P S the present example, considerable improvements have been made in the growth of grains. In correspondence thereto, the M12 0'010 060 mechanical properties have been substantially improved TABLE 2(b).MECHANICAL PROPERTIES Yield Tensile Elonga- Mean Mean value point, strength, tion, value of Ericksen C.C.\ Charge kg./mm. kgJrnrn. percent of 7 Tests, mm. mm.

13" 15.8 28. 7 52. o 1. 71 I2. 0 as. 6

TABLE 1(b) Mechanical Properties of Steel Sheet Products Mean Yield Tensile Elonga- Mean value of point, strength tion, value of Erichsen C.C.V., Charge Kind of heat treatment kg./mm. kg./mrn. percent 7 tests, mm. mm. A th fiitriitllinti: 53:3 33:? 538 {it i818 2%? 800 C. docnrburizod 18.6 29.8 51.2 1. 50 11.6 37.0

NOTE.-Tll0 thickness of the steel sheet amounted to 0.8 mm.

By adoping the aforementioned improving measure, it could be feared however that in the course of annealing at higher temperature the crystalline grains would become excessively larger or roughened. In FIG. 3, a microscopic photograph Now taking an example wherein the carbon content in steel is 0.1% and the steel is kept at 800 C for resonable time period, 'y-phase occupying generally in this case about onefourth the total weight of steel, when the decarburizing treatf the Steel sheet fl mechanical p p f as 5 ment is initiated from this condition, the occupation of aal'ldconlltloned thrPugh annfialmgai 9 phase relative to 'y-phase will be gradually increased and a C 15 glven- T1115 Steel 15 ldemlfied therem by In the Similar considerable reduction of 'y-phase or disappearing thereof will gi 25?: ll Sed d f h d h t occur. Under extreme circumstances it will bringatransfer of S m we SUPPO m o mug ee rain boundaries formed between aand hase, thereb the frequently represem disadvantageous orallgepeelings 10 grain growth in 'y-phase being considerabl y affected withi zonfi s g i fi gi zi s g; i i gg z pi g to Severe siderable possibilities. Therefore, it can be supposed that at The annealing process iii the ai t is c oiiim cjiily i ari'ied out in h of glsappearmg y-phasg having a 33;

si era i er car on content, e ex ecte rain 0 the furnace the batch type will be iiivitfd. From observation of FIG. 3 showi ng co rse or 3:2 izi gszg fi ggfiggfiz iigz g12:21:25? 5 3 3; 15 roughened grain sizes which are believed to be caused to take this case however part of the cdil may frequ ntly elevated 10- plaza from a segondzry recrystghzauon' we have supligsed w cally In its temperature, to such a degree enough to roughen :g :22:2 i z g i figg zg f g g i gi the crystal grains in the corresponding area thus affected. If an tioned reduction or disappear 2 Twas: we have funher im roved measure for controllin the c stalline structure so u n as 10 create and maintain subsantiall y equally distributed g g suppgsmon be true and 8? steel at a temperature e ow trans ormedium size rains inastabilized manner, the aforementioned ls F {an ecar unze y for pfi g the g g tendency of crystal grains matron point A,, a decarburizing treatment at an intermediate through the way of reducing the aforementioned minor parti- F g between 1 a czuld {wt nng m none of cles in their unit number can be relied upon with safe y to a 25 apprlacla e or eiicesslve egree o gram-Size gm or mot-e satisfyin de ree specifically the disadvantageous roughening. In other words, if

g the carbon content in the form of Fe C is removed to a satisfy- We have investigated into the aforementioned phenomenon in de ee at a tem mature below transformation in A still further in the above-mentioned direction and brought the carbu treatment is carried out az about an improved process for the manufacture of steel sheets t :2 abru t comngin or size roughenin will take lace with superior deep-pressing qualities through the way of a P bl d s indeed in conmgm to thatpcase higher temperature annealing of the said conditioned steel mhany appzecla i d rblfizi win ccur B aus sheets, yet without any risk of developing excessively coarse ere S mg ng o ec e or roughened crystal grains. 7 1n the former case, 'y-phase is already of smaller percentage More specifically to explain, a comparison between the and thus egndthmugh m g gg i l main features of both steels A and B enlisted in the phas? Shou lsappear ere Straws a micro foregoing Tables I and 2, will give the following data: V SQPYP Showmg the i Structure of l Fear of exees iliqvet IOW' O W Carbon Oxygen Minor Grain growth Mechanical grains at kind of steel in steel in steel particles performance properties higher temp.

A" 0.05 0.060 Much Bad Better None. "B" 0.12 0,029 Best. Feared.

Small Good.

In steel B, it includes twice a high percentage of carbon relative to that contained in steel A, for the purpose of reducing the said minor particles and thus for improving the. performance in grain size growth. As is commonly known, cold-rolled steel sheet products are generally subjected to the open coil annealing for the purpose of realizing a substantially complete decarburizing, it can be said that there would be no practical difference in carbon content between the both kinds of steel A" and B, so far as the final products are concerned. Generally speaking, regular decarburizing annealing is carried out at a suitably selected temperature directly below the transition point A,. In this case, when the products contain a higher percentage of carbon in steel, a longer extended time period would be necessary to elapse before the termination of a complete decarburizing treatment, while it be allowed to provide a mediumly extended soaking period, the grain size may be subjected to growth substantially to its maximum allowable degree at the specified temperature. Therefore, in this case, the higher carbon content in steel Would affect in no direct way upon the grain size as appearing at completion of the annealing treatment.

Now considering such a case wherein the decarburizing is carried out at an intermediate temperature between A and A there are a-phase and 'y-phase kept in their co-existence in the steel, y-phase being naturally of austenite, the decarburizing process will progress accompanied by the corresponding reduction of y-phase, until the carbon content will become a value lower than the solid-soluble limit in the a-phase specific to the temperature under consideration.

B which has been decarburized at 700 C until the carbon content is reduced to 0.04% and then the decarburizing conditioning is continued at 800 C so as to provide a final carbon content of 0.002% by weight. In this case, as we expected, a decarburizing treatment of the steel at 800 C did not lead to excessive roughening of grain size in any appreciable degree. We have further investigated into the conditioning treatment based upon these data and relative to a large number of steels having various and different compositions in the above sense, and found out that even when the decarburizing treatment is carried out at intermediate temperatures between A and A while specifying the carbon content at the intiating point of the treatment to 0.03% or less, the disadvantageous coarse grain formation or roughening can be effectively prevented. It should be noted in this case, however, the above specified carbon content of 0.03% is an example of satisfying value for the effective prevention of excessive grain size growth. Therefore, under circumstances a higher carbon content of 0.06% did not invite a roughening of grain size. Based upon the aforementioned experimental data and the analyzed novel teaching, we ,have ascertained proper treating conditions and the mechanical properties of the final products, the following processing conditions could provide an improved process for the manufacture of thin sheets with superior deep-pressing qualities, said process being realized in such a way that the steel charge is conditioned properly in the course of the steel manufacture so as to favorably affecting the grain growth performance as appearing at the time of later decarburizing annealing, thus p pa tenshe ss em els ste sa set which are hot-rolled, cold-rolled, and then subjected to a decarburizing EXAMPLE 2. treatment at a moderate temperature below transformation A t l h h f point A, until the carbon content of steel will be reduced to ea c arge avmg a composmon set mm m the follow ing Table 4(a) was prepared and processed as before to cold- 0.06% or less, preferably 0.03%. 5 drawn Sheds 08 mm thick- In the following, several preferred embodiments of the invention will be given only by way of example. For carrying out TABLE 4(a the process according to this invention, steel charges having a carbon content 0.06 0.20%; a manganese content 0.20 Chemical Composition 0.80; least possible contents of P, S and the like, may

preferably be used. The contents of carbon and manganese 10 D 3*, 0.033 should be so conditioned that the oxygen content in the molten steel be lowest as possible, yet in consideration of the Th h ldll d il w charge i an annealing f desired rimming action as well as the stabilized Working 35 a nace, and when the coil temperature reached 500 C, th f whole. In consideration'of the balance with the oxygen connace atmosphere was conditioned so as to show a decarburiztent, carbon content should preferably be reduced to a ing nature, then the temperature was increased to 800 C in reasonable value when manganese content be higher. Recomh course of 4 hours as h degarburizing process r d d, mended hot rolling condition should preferably be 800- 890 At C, a oaking process f further 4 hours was con- C in terms of finishing temperature. Wind-up temperature ti d, d a dry ki g for an hour, whereupon the products ma be 550 700 C and cold-rolling rate should preferably were cooled to room temperature. A refining or conditioning be 50 80%. rolling was carried out by about 1.0 The mechanical properties of the thus prepared products are shown in the follow- EXAMPLE ing Table 4(b), while a microscopic photo of the correspond- A molten steel charge, named having a composition as iase ystal l a yqwt as wn m Q- 7 set forth in the following Table 3(a) has been prepared, TABLE 4(b CHEMICAL COMPOSITION processedasconventionallyintoingots. Yield Tens- Elonga Mean oint, str., tion, value Erick, C.C.V., Steel kgifinm. kgn/rmn. percent of 1 mm. mm.

TABLE 3(a) 13".... 15.7 28.5 52.2 1. 72 12.3 36.3 c M11 P s 0 N, W xnfwm l n V w l 0.14042 (mm Q92] "0:09,! 0* From the foregoing Examples 1 and 2, it will become clear that according to the present invention, cold-rolled steel sheets having grain size corresponding to A.S.T.M. No. 6 or 5, superior results substantially equal to, or even more superior results can be obtained, in comparison with those provided These ingots were at first hot-rolled at a finishing temperature by A I-ki1led steel sheets. of 860 C' or at a wind-up temperature 570 C, th ll d It is to be understood that theabovedescr bed examples are products being 2.3 mm thick, where n the hotirollgd bani only illustrative of the apphcation of the principles of the inwere picked and passed through atan em millfor cold-rolling VeHtIOIL A numbelf 0 fihang s and modifications may be them to final size of 0.8 mm. The thus produced sheets were 40 f't y those Skllleq an wlthout dePartmg from the decarburizingly annealed under the following conditions, p t' and Scope of the lnventlon as Set forth the pp cooled to room temperature and then subjected to a refining claims-i rolling by about 10%. e Claim: I D

At first stage: 1. Ifrocess for the manufacture of cold-rolled steel strip with decarburizing anneal at 700 C f 3 hours; superior mechanical workability, especially deep-forming pro- At second stage: pert es, comprising the steps of:

decarburizing anneal at 800 C for 4 hours; subjected to P p a steel'chal'ge mcludmg a carbon Content of d ki at 800C f 1 h 0.06 to 0.20 wt%, and a manganese content of 0.20 to The mechanical properties of the thus obtained cold-rolled Wt 7 I sheets are shown in the following Table 3(b), a microscopic p p f cold-rqlled Steel F p from 531$! Charge photograph of the crystalline structure of a sample taken P gi hot-foll'ltlg and Cold-711mg under therefrom, is shown in FIG. 6. When comparing this photo- Y t ly p y q P graph with that shown in FIG. 5 which illustrates the structure Subjectmg Sald f P to a prellmmary anflealmg at a of the same composition of steel, yet subjected to a decarbu- Petature of 7 that temperature bemg P Q the r rizing treatment at 750 C, the former represents a highly statl'ansfPrmatlon Polnt to effect thus bilized and well-distributed crystalline structure, which means reducing the calbon Content of 531d Strlps to less than a considerable improvement in the art. (106%; and

cl. further subjecting said decarburized strips to a second decarburizing annealing at a temperature of 800 C., said TABLE 3(b).MEOHANICAL PROPERTIES temperature being between the A -transformation point Yielding Tensile Elongag Mean Ericksen and the A -transformation point, thus reducing said strips point, strength, tion, value test, O.C.V., to the required carbon content, while preventing the for- Charge kglmmf Percent T mation of a coarsened grain structure. 

